Respiratory syncytial virus (RSV) is the most important respiratory pathogen in infancy and early childhood and is associated with the development of long-term airway inflammation and hyperreactivity whose mechanisms remain unclear. Our recent research (HL-61007) has shown that RSV makes rat airways abnormally susceptible to neurogenic inflammation through mechanisms involving the increased expression of nerve growth factor (NGF) and its trkA and p75 receptors, and we have confirmed overexpression of neurotrophic factors [NGF and brain-derived neurotrophic factors (BDNF)] and trkA receptors in the lower airways of human infants with RSV bronchiolitis. We also found that the recruitment of lymphocytes and monocytes to infected airways is amplified by concomitant stimulation of substance P (SP)-containing sensory afferents and that RSV induces maturation of large numbers of mast cells in close juxtaposition with these nerve fibers, whose activation triggers the synthesis/release of leukotrienes in the airways. Furthermore, our studies indicate that the potentiation of neurogenic inflammation persists long after resolution of the acute RSV infection and that neurogenic responses to RSV in young rats are qualitatively and quantitatively different from adult rats. Starting from these observations, we now hypothesize that dysregulation of the expression of neurotrophins [NGF, BDNF, and neurotrophins (NT) 3 and 4) and their receptors (trkA/B/C and p75) in developing airways by early-life RSV infection leads to remodeling of the sensorineural network in the respiratory tract and to development of complex neuro-immune interactions that amplify and protract airway inflammation and hyperreactivity. We also hypothesize, based on preliminary data, that exposure to hyperoxia in the neonatal period modifies the effect of the virus on the expression of neurotrophic factors and receptors. Finally, we hypothesize that RSV-induced changes in neurotrophin expression are responsible for the aberrant responses of cellular effectors of inflammation and allergy to the stimulation of sensory nerves. The following specific aims are organized around the evaluation of these hypotheses: 1) To study the changes in expression and localization of neurotrophins and neurotrophin receptors in the lungs resulting from early-life RSV infection, and correlate these changes to neurogenic inflammation;2) To determine the role of neurotrophins and neurotrophin receptors in the recruitment and activation of lymphocytes and monocytes into the respiratory tract infected with RSV;and 3) To analyze the role of neurotrophins and neurotrophin receptors in the differentiation of mast cells and modulation of the leukotriene pathway in the respiratory tract infected with RSV. These studies will provide new important information concerning neuro-immune interactions in RSV-infected airways and on the pathogenesis of post-RSV childhood asthma, and may lead to new therapeutic and prophylactic strategies.